Home About us Contact
|
Home About us Contact | ||
|
|
||
Transition Energies (transition + energy)
Selected AbstractsThe electronically excited states of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine): Vertical excitationsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 11 2009Itamar Borges Jr Abstract The RDX molecule, hexahydro-1,3,5-trinitro-1,3,5-triazine, is a key component for several energetic materials, which have important practical applications as explosives. A systematic study of the electronic excited states of RDX in gas phase using time-dependent density functional theory (TDDFT), algebraic diagrammatic construction through second order method [ADC (2)], and resolution of the identity coupled-cluster singles and doubles method (RI-CC2) was carried out. Transition energies and optical oscillator strengths were computed for a maximum of 40 transitions. RI-CC2 and ADC (2) predict a spectrum shaped by three intense ,-,* transitions, two with charge transfer and one with localized character. TDDFT fails in the description of the charge transfer states. The low-energy band of the experimental UV spectrum of RDX is assigned to the first charge transfer state. Two alternative assignments of the high-energy band are proposed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Theoretical study on the second-order nonlinear optical properties of nonconjugated D-,-A chromophoresINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 7 2009Guochun Yang Abstract Density functional theory calculations have been carried out on nonconjugated D-,-A chromophores to investigate the different electron donors and conjugated bridges effects on the molecular nonlinear optical response. The results show that the large second-order polarizability values can be achieved through careful combination of available electron donors, conjugated bridges for our studied nonconjugated D-,-A chromophores. The calculations also provide a clear explanation for the second-order polarizability changes from the standpoint of transition energies, oscillator strengths, electron density difference, and bond length alternation. Solvent effect has great influence on the second-order polarizability and electronic absorption spectrum. It is hoped that the results presented in this article will give some hints to the interrelated studies. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Theoretical electronic spectra of 2-aminopurine in vapor and in waterINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 13 2006Antonio Carlos Borin Abstract The accurate quantum chemical CASSCF and CASPT2 methods combined with a Monte Carlo procedure to mimic solvation effects have been used in the calculation of the spectroscopic properties of two tautomers of 2-aminopurine (2AP). Absorption and emission spectra have been simulated both in vacuum and in aqueous environment. State and transition energies and properties have been obtained with high accuracy, leading to the assignment of the most important spectroscopic features. The lowest-lying 1(,,,*) (1La) state has been determined as responsible for the first band in the absorption spectrum and also for the strong fluorescence observed for the system in water. The combined approach used in the present work gives quantitatively accurate results. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Intramolecular electronic communication in a dimethylaminoazobenzene,fullerene C60 dyad: An experimental and TD-DFT studyJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2010K. Senthil Kumar Abstract An electronically push,pull type dimethylaminoazobenzene,fullerene C60 hybrid was designed and synthesized by tailoring N,N -dimethylaniline as an electron donating auxochrome that intensified charge density on the ,-azonitrogen, and on N -methylfulleropyrrolidine (NMFP) as an electron acceptor at the 4 and 4, positions of the azobenzene moiety, respectively. The absorption and charge transfer behavior of the hybrid donor-bridge-acceptor dyad were studied experimentally and by performing TD-DFT calculations. The TD-DFT predicted charge transfer interactions of the dyad ranging from 747 to 601 nm were experimentally observed in the UV-vis spectra at 721 nm in toluene and dichloromethane. A 149 mV anodic shift in the first reduction potential of the NN group of the dyad in comparison with the model aminoazobenzene derivative further supported the phenomenon. Analysis of the charge transfer band through the orbital picture revealed charge displacement from the n(NN) (nonbonding) and , (NN) type orbitals centered on the donor part to the purely fullerene centered LUMOs and LUMO+n orbitals, delocalized over the entire molecule. The imposed electronic perturbations on the aminoazobenzene moiety upon coupling it with C60 were analyzed by comparing the TD-DFT predicted and experimentally observed electronic transition energies of the dyad with the model compounds, NMFP and (E)-N,N -dimethyl-4-(p-tolyldiazenyl)aniline (AZNME). The n(NN) , ,*(NN) and ,(NN) , ,*(NN) transitions of the dyad were bathochromically shifted with a significant charge transfer character. The shifting of ,(NN) , ,*(NN) excitation energy closer to the n , ,*(NN) in comparison with the model aminoazobenzene emphasized the predominant existence of charge separated quinonoid-like ground state electronic structure. Increasing solvent polarity introduced hyperchromic effect in the ,(NN) , ,*(NN) electronic transition at the expense of transitions involved with benzenic states, and the extent of intensity borrowing was quantified adopting the Gaussian deconvolution method. On a comparative scale, the predicted excitation energies were in reasonable agreement with the observed values, demonstrating the efficiency of TD-DFT in predicting the localized and the charge transfer nature of transitions involved with large electronically asymmetric molecules with HOMO and LUMO centered on different parts of the molecular framework. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] TDDFT investigation on nucleic acid bases: Comparison with experiments and standard approachJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2004M.K. Shukla Abstract A comprehensive theoretical study of electronic transitions of canonical nucleic acid bases, namely guanine, adenine, cytosine, uracil, and thymine, was performed. Ground state geometries were optimized at the MP2/6-311G(d,p) level. The nature of respective potential energy surfaces was determined using the harmonic vibrational frequency analysis. The MP2 optimized geometries were used to compute electronic vertical singlet transition energies at the time-dependent density functional theory (TDDFT) level using the B3LYP functional. The 6-311++G(d,p), 6-311(2+,2+)G(d,p), 6-311(3+,3+)G(df,pd), and 6-311(5+,5+)G(df,pd) basis sets were used for the transition energy calculations. Computed transition energies were found in good agreement with the corresponding experimental data. However, in higher transitions, the Rydberg contaminations were also obtained. The existence of ,,* type Rydberg transition was found near the lowest singlet ,,* state of all bases, which may be responsible for the ultrafast deactivation process in nucleic acid bases. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 768,778, 2004 [source] Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2002Xiang-Yuan Li Abstract In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor,acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state 1A1 and in lowest triplet state 3B1, but charge separation appears in excited singlet state 1B1. Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, ,,,* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 874,886, 2002 [source] Optical anisotropy of A - and M -plane InN grown on free-standing GaN substratesPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 5 2010P. Schley Abstract Wurtzite A - and M -plane InN films were grown by molecular beam epitaxy (MBE) on free-standing GaN substrates. Spectroscopic ellipsometry (SE) in the photon energy range from 0.56 up to 15,eV was applied in order to determine the ordinary and extraordinary complex dielectric function (DF) of InN. A distinct optical anisotropy was found over the whole energy range. The extraordinary absorption edge in comparison to the ordinary one is shifted to higher energies confirming previous studies. The investigations in the upper vacuum-ultraviolet (VUV) spectral range (9.5,15,eV) yielded transition energies for four critical points (CPs) of the band structure (BS) which have not been observed so far. [source] Valence-band splitting and optical anisotropy of AlNPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2010G. Rossbach Abstract The dielectric function (DF) of hexagonal AlN on Si(111) is determined in the range between 1 and 9.8,eV by spectroscopic ellipsometry (SE). Due to its large negative crytal-field splitting wurtzite AlN features large dichroism. Showing that SE is sensitive to both components of the DF around the absorption edge, a uniaxial model is applied which yields transition energies for the free excitonic state. The in-plane tensile stress leads to a red-shift of these transitions and to an enlarged splitting. The experimental data are compared to the results of band-structure calculations demonstrating excellent overall agreement. In addition, two high-energy critical points in the ordinary DF were determined at energies of about 7.75 and 8.85,eV. [source] Effects of composition distribution on electronic structures of self-assembled InGaN/GaN quantum dotsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2010Wei-Yi Tsai Abstract In this study, we investigate the influences of different composition distributions on the electronic structures of truncated cone-shaped InGaN quantum dots (QDs). A varying parameter, , is defined as , in which w1 and w2 are the top and base diameter of QD, respectively. This factor is set to study the role of similar truncated cones with the same base diameter and height on electronic structures of QD. Three indium composition distributions are considered: ellipsoid, uniform, and linear. The single-band effective-mass equation and six-band k·p theory are used to calculate the transition energies of electrons and holes, respectively. The numerical results reveal that the parameter plays an important role in changing the piezoelectric potential. In addition, an InGaN quantum dot with a linear distribution of indium has the maximum transition energy, whereas one with an ellipsoidal distribution has the minimum value. It is noteworthy that the binding energy greatly decreases as increases for different indium distributions. [source] Theoretical study of the electronic structure and the totally symmetric vibrations of selected CoMoCat carbon nanotubesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2008Kürti Abstract In situ Raman spectroelectrochemical studies of CoMo-Cat single-walled carbon nanotubes enriched in (6,5) tubes have been carried out recently. We performed calculations on the density functional level using local density approximation for the electronic and vibrational properties of the most abundant tubes in these samples. The following chiral semiconducting tubes were investigated: (6,4), (7,3), (6,5), (9,1), (8,3) and (7,5). The calculated and the measured frequencies of the RBM and G, modes agree within several wave numbers. The calculated E11, E22 transition energies -after 30% and 20% upscaling, respectively- are comparable with the experimental values. The quenching of the RBM band with p- and n-doping can be interpreted within the rigid band approximation. The validity of the rigid band approximation was shown by calculating the density of states for neutral and charged tubes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The role of environmental effects on the optical transition energies and radial breathing mode frequency of single wall carbon nanotubesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2008P. T. Araujo Abstract In this paper we discuss the environmental effects on the radial breathing mode (RBM) spectra of Single Wall Carbon Nanotubes (SWNTs). We have shown that the environmental effect on the radial breathing mode frequencies can be explained by Van-der-Waals interactions. We here explore these interactions concerning its (n,m) dependence and curvature effects. Furthermore, most of the optical transition energies available in the literature (ELit.ii) are downshiftedwith respect to the optical transition energies for super-growth (S.G.) tubes (ES.G.ii). The effect on transition energies can be understood considering the effect of the dielectric constant of the medium in the excitonic optical transition. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Optically induced strain relaxation in anisotropically strained M -plane GaN filmsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 5 2008T. Flissikowski Abstract We study the anisotropic in-plane strain in M -plane GaN films by photoreflectance, photoluminescence, Raman, and time-resolved pump-and-probe spectroscopy. We find that a highly strained film partially relaxes, if it is pumped by an intense optical pulse. The strain relaxation can be observed by a shift of the E2 -Raman line to lower energies and by a shift of the fundamental interband transition energies in the photoreflectance spectra. The photoluminescence intensity of the exposed areas is significantly reduced as compared to the one for areas, which have not been exposed to the intense optical pulse. This suggests that the strain relaxation is connected to the introduction of defects, which can act as non-radiative recombination centers. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] First-principles study of electronic structures of CdMoO4 crystal containing F-type color centersPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2008Xi'en Wang Abstract The electronic structures of the CdMoO4 crystal containing F-type color centers are studied within the framework of the fully relativistic self-consistent Dirac,Slater theory, using a numerically discrete variation (DV-X,) method. It is concluded from the calculated results that the F and F+ color centers have donor energy levels in the band gap and the optical transition energies are 1.761 eV and 1.941 eV, which correspond to the 706 nm and 640 nm red absorption bands, respectively. So the CdMoO4 crystal should exhibit a blue-green color. It is predicted that the 650,750 nm absorption bands and the bluish green color originate from the F and F+ color centers in the CdMoO4 crystal. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Raman intensities of the first optical transitions in carbon nanotubesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 13 2006H. Telg Abstract We performed resonant Raman spectroscopy on the lowest optical transition ES11 of separated single-walled carbon nanotubes by studying the radial-breathing mode (RBM) spectra for excitation energies between 1.15 and 1.48 eV. We were able to extend the experimental Kataura plot to these energies by adding the ES11 transition energies of 11 nanotube chiralities. We discuss also the relative Raman intensities; they are more similar for different family index , than those of the corresponding transitions of the ES22 [1]. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Hydrostatic-pressure effects on the correlated electron,hole transition energies in GaAs,Ga1,xAlxAs semiconductor quantum wellsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2006N. Raigoza Abstract The effects of hydrostatic pressure on the correlated e,h transition energies in single GaAs,Ga1,xAlxAs quantum wells are calculated via a variational procedure, in the framework of the effective-mass and non-degenerate parabolic-band approximations. The valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. Both heavy- and light-hole exciton energies are obtained, and correlated e,h transition energies are found in good agreement with available experimental measurements. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic Properties of Propylamine-Functionalized Single-Walled Carbon NanotubesCHEMPHYSCHEM, Issue 11 2010Matthias Müller Abstract We present resonant Raman measurements on single-walled carbon nanotubes (SWCNT) functionalized with propylamine groups at different degrees. Direct nucleophilic addition based on in situ generated primary amides is used for attaching n -propylamine to the sidewalls of SWCNTs. The influence of the amino functionalities on the electronic structure of the nanotubes is investigated. From the Raman resonance profiles of the radial breathing modes (RBMs), the chiral indices of the corresponding tubes are assigned. We observe significant redshifts of the transition energies and a broadening of the resonance windows due to chemical modification of SWCNTs. Similar redshifts are derived from the analysis of the NIR/Vis transmission spectrum. The relative Raman intensities of the functionalized samples and the evaluation of their transmission spectra indicate a diameter dependence of the reactivity as it has been observed for other moieties. By analyzing the defect induced D mode we observe a considerable degree of functionalization accompanied by an almost unharmed tube structure, which ensures that the observed effects are mainly driven by changes of the electronic structure. [source] The Low-Lying Excited States of 2,2,-Bithiophene: A Theoretical AnalysisCHEMPHYSCHEM, Issue 12 2003Mercedes Rubio Dr. Abstract The low-energy regions of the singlet,singlet, singlet,triplet, and triplet,triplet electronic spectra of 2,2,-bithiophene are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 11Bu and 21Bu, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 23Ag and 23Bu, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states. [source] Absolute configuration of eremophilane sesquiterpenes from Petasites hybridus: Comparison of experimental and calculated circular dichroism spectraCHIRALITY, Issue 3 2010Antje Bodensieck Abstract In-depth conformational analyses of 10 known eremophilane (= (1S,4aR,7R,8aR)-decahydro-1,8a-dimethyl-7-(1-methylethyl)napththalene) sesquiterpenes, 1,10, from Petasites hybridus were performed with molecular mechanics as well as density functional theory methods. Electronic transition energies and rotational strengths of these eight eremophilane lactones and two petasins were calculated by time-dependent density functional theory (B3PW91/TZVP). The absolute configurations of the constituents could be assigned by comparison of their simulated and experimental circular dichroism (CD) spectra in methanol as (4S,5R,8S,10R) (1, 2), (2R,4S,5R,8S,10R) (3, 4, 5), (2R,4S,5R,8R,9R,10R) (6), (2R,4S,5R,8R,10R) (7, 8), and (3R,4R,5R) (9, 10). Single-crystal X-ray diffraction data of 8,-hydroxyeremophilanolide ((8S)-8-hydroxyeremophil-7(11)-en-12,8-olide) (1) served as starting point for the theoretical conformational calculations of the 8,-epimers of the eremophilane lactones. Experimental CD spectra as well as 1H NMR spectra of compound 1 in methanol were considerably dependent on sample concentration. Chirality, 2010. © 2009 Wiley-Liss, Inc. [source] Calculation of the electronic structure of AmO2 and Pr6O11 for XANES analysis with redox propertyINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 12 2009Chikashi Suzuki Abstract We calculated X-ray absorption near-edge structure (XANES) of Am LIII of AmO2 and Pr LIII of Pr6O11 using the relativistic discrete-variational (DV)-X, method based on the Dirac,Slater method, and compared it with the experimental spectra. These spectra were calculated on a model of AmO2 ([AmO8]12,) and Pr6O11 ([PrO8]12,). In spite of using small cluster models, the calculated spectra were in good agreement with the experiment ones. Besides, we calculated the electronic structure of AmO2 and Pr6O11 to analyze the peak structures of XANES. From this calculation, it was found that O s, p, and f components had influence on the specific peak structures but that O d component had influence on various peak structures for AmO2 and Pr6O11. From this result, it was suggested that the change of the electronic structure of actinide 6d and O d or f was important for actinide LIII XANES corresponding to oxygen to metal ratio in the oxide nuclear fuel. On the basis of these results, we calculated the fine structures of densities of states and the transition energy from the HOMO to the white line of AmO2, UO2, and Pr6O11 and investigated redox properties of Am and U in the oxide nuclear fuel with the evaluation of validity of Pr as simulant materials of Am. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Structural characterization of the ternary solvent mixture methanol,acetonitrile,1-propanol,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 9 2002Ruben Elvas Leitão Abstract Refractive indices and ET values were measured for the ternary mixture methanol,acetonitrile,1-propanol at 25.0,°C for 13 mole fractions, and also for the corresponding binary mixtures, methanol,1-propanol, methanol,acetonitrile and 1-propanol,acetonitrile, at 25.0 and 50.0,°C, at 10 different compositions. Solvent exchange equilibrium models were applied to the transition energy of the Dimroth,Reichardt ET(30) solvatochromic indicator in the binary systems and the Redlich,Kister polynomial was used to correlate excess ETN and nD values for the binary solvent mixtures data. The results allowed the analysis of synergetic behaviours, polarizability effects and preferential solvation trends both in the binary and in the ternary mixtures. Our results point towards the prevalence of specific solute,solvent,solvent interactions mainly due to hydrogen bonding by the hydroxylic components of the ternary mixture. Copyright © 2002 John Wiley & Sons, Ltd. [source] Calculation of XANES/ELNES Spectra of All Edges in Si3N4 and Si2N2OJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 1 2002Wai-Yim Ching Using a recently developed first-principles supercell method that includes the electron and core-hole interaction, the XANES/ELNES spectra of Si- L2,3, Si- K, and N- K edges in ,-Si3N4, ,-Si3N4, spinel c -Si3N4, and Si2N2O were calculated and compared. The difference in total energies between the initial ground state and the final core-hole state provides the transition energy. The calculated spectra are found to be in good agreement with the experimental measurements on ,-Si3N4 and c -Si3N4. The differences in the XANES/ELNES spectra for the same element in different crystals are explained in terms of differences in local bonding. The use of orbital-decomposed local density of states to explain the measured spectra is shown to be inadequate. These results reaffirm the importance of including the core-hole effect in any XANES/ELNES spectral calculation. [source] Effects of composition distribution on electronic structures of self-assembled InGaN/GaN quantum dotsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 7 2010Wei-Yi Tsai Abstract In this study, we investigate the influences of different composition distributions on the electronic structures of truncated cone-shaped InGaN quantum dots (QDs). A varying parameter, , is defined as , in which w1 and w2 are the top and base diameter of QD, respectively. This factor is set to study the role of similar truncated cones with the same base diameter and height on electronic structures of QD. Three indium composition distributions are considered: ellipsoid, uniform, and linear. The single-band effective-mass equation and six-band k·p theory are used to calculate the transition energies of electrons and holes, respectively. The numerical results reveal that the parameter plays an important role in changing the piezoelectric potential. In addition, an InGaN quantum dot with a linear distribution of indium has the maximum transition energy, whereas one with an ellipsoidal distribution has the minimum value. It is noteworthy that the binding energy greatly decreases as increases for different indium distributions. [source] Determination of the band-gap of MgS and MgS-rich Zn1,xMgxSySe1,y alloys from optical transmission measurementsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2010Ian A. Davidson Abstract As part of our development of an epitaxial lift-off process, utilising a sacrificial magnesium sulphide (MgS) layer, we have developed a MgS-rich ZnMgSSe alloy which provides excellent carrier confinement and resists both oxidation and acid attack. Here the optical transmission of the alloy has been measured and its bandgap determined as a direct transition at 4.19,±,0.04,eV. Its composition has also been determined by X-ray interference (XRI) and comparison with simulations. For a range of alloy samples we obtain compositions of the Zn1,xMgxSySe1,y layers which are (x, y),=,(0.80,±,0.02, 0.645,±,0.025). Using the alloy bandgap and composition we have determined direct bandgap transition energy for MgS by extrapolation. This is found to be 4.78,±,0.14,eV. [source] Shape-dependent properties of self-organized quantum dots: Few-particle states and exciton-phonon couplingPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003R. Heitz Abstract The electronic and optical properties of self-organized InAs/GaAs quantum dots are investigated in view of the actual structural properties. Focussing on the shape we demonstrate theoretically a strong impact of the varying strain distribution on the electronic and, in particular, the exciton properites in such quantum dots. The inhomogeneous strain, typical for self-organized quantum dots, lowers the symmetry and increases the local charge density. Resonant Raman experiments on pyramidal InAs/GaAs quantum dots show an enhance exciton,LO-phonon coupling reflecting the strain-induced local charge density. Experiments on single InAs/GaAs quantum dots demonstrate an unexpected strong impact of the structural properties on few-particle complexes on the example of the biexciton complex. Anti-binding biexciton complexes are demonstrated. The apparent correlation of the biexciton binding energy and exciton transition energy, with a transition from binding to anti-binding at ,1.24 eV, is attributed to a decreasing exciton localization, resulting from the finite barrier height. The Coulomb interaction with delocalized states is negligible for the energy of the localized states. [source] Discrete luminescence bands in AlGaN-based quantum wellsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2009Hideaki Murotani Abstract The excitation-power-density dependent photoluminescence (PL) spectra have been studied for AlGaN-based quantum wells (QWs) with a well-layer thickness of 2, 4, and 6 nm. With increasing excitation-power density, an additional luminescence line was observed at the higher energy side of an initial luminescence line for the QWs with the well-layer thickness of 4 and 6 nm. The additional line also shifted toward higher energy side with further increasing excitation-power density. It was found from a theoretical calculation of the transition energy under applied electric field that an energy difference between the additional and the initial lines agreed with an energy separation for a well-layer-thickness variation of 2 monolayer for each QW. Therefore, these observations indicated that in the QWs with the well-layer thickness of 4 and 6 nm, the screening of the internal electric field occurred effectively after the saturation of localized states caused by the interface disorder. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Infrared transient absorption spectra for excited transition of excitons and biexcitons in CuClPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2009Takaaki Yoshioka Abstract Infrared transient absorption (IRTA) spectra for excitons and biexcitons in CuCl bulk crystal have been measured by pump-probe spectroscopy. The IRTA peak energy of the exciton agrees with the transition energy between Rydberg 1s to 2p states reported in one- and two-photon absorption measurement. On the other hand, the IRTA peak energy of the biexcitons locates in energy higher than that of the excitons, which is reasonable by taking hydrogen molecule model into account. In addition, our results support the enhancement of the transition energy between 1s and 2p states in quantum dots compared to the bulk case. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Excited states and spontaneous transition lifetimes of donor impurities in quantum dotsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2007G. Murillo Abstract We calculate the 2p,1s-like transition energy and the spontaneous lifetime of an on-center shallow donor impurity in a spherical parabolic GaAs-(Ga,Al)As quantum dot (PQD) as a function of the radius of the structure and the strength of an applied electric field. In our calculations we use a variational method, within the effective mass and dipolar approximations. We find that the spontaneous lifetimes increase with the radius of the PQD and the applied electric field. In this direction our results revel that the electric field can be used to suppress the electron-phonon interaction driving to the increasing of the 2p-1s spontaneous life-time, showing the feasibility of using impurity states to be used in quantum computing developments. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic structure of three-dimensional triangular torus-shaped quantum rings under external magnetic fieldsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2003Yiming Li Abstract In this paper, we calculate the electron,hole energy states and the magnetization for InAs/GaAs triangular torus-shaped (TTS) quantum rings in a magnetic field. Our three-dimensional (3D) model considers (i) the effective one-band Hamiltonian approximation, (ii) the position- and energy-dependent quasi-particle effective mass approximation, (iii) the finite hard wall confinement potential, and (iv) the Ben Daniel-Duke boundary conditions. This model is solved numerically with the nonlinear iterative method to obtain the "self-consistent" solutions. We investigate the electron-hole energy spectra versus magnetic field for two different ring widths: R0 = 20 and 50 nm, and find that they strongly depend on the ring shape and size. Since the magnetic field penetrates into the inside region of the nonsimply connected ring, the electron (hole) transition energy between the lowest states versus magnetic field oscillates nonperiodically and is different from that of quantum dots. We find the magnetization at zero temperature is a negative function, saturates, and oscillates nonperiodically when the magnetic field increases. [source] Piezoelectric Field Influence on GaN/AlxGa1,xN Quantum Well Optical PropertiesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2003S. Fanget Abstract The absorption and luminescence properties of hetero-polarization GaN/AlxGa1,xN (x = 0.12 and 0.165) quantum well (QW) structures are studied by photoreflectivity, photoluminescence excitation spectroscopy (PLE), and photoluminescence at low temperature. The QW transition energy as a function of well thickness exhibits a quantum-confined Stark effect (QCSE) due to the presence of a strong built-in electric field (piezoelectricity and spontaneous polarization). An electric field strength of 120 kV/cm in the barrier and between 600 and 800 kV/cm in the well are obtained from the analysis of Franz-Keldysh oscillations in photoreflectivity spectra. These values are in good agreement with results from the fit of the QW transition energy versus the thickness, using the electric field as a parameter. [source] Novel copper(II) heterochelate: synthesis, structural features and fluorescence studiesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 7 2010G. J. Kharadi Abstract Fluorescence properties of four based derivatives [An] (where n = 1,4) and their Cu(II) heterochelates of the type [Cu(An)(CQ)(OH)],xH2O {where A1 = 3-(2-oxo-2H-chromen-3-yl)-4H-furo[3,2-c]chromen-4-one, A2 = 8-methyl-3-(2-oxo-2H-chromen-3-yl)-4H-furo[3,2-c]chromen-4-one, A3 = 6-methyl-3-(2-oxo-2H-chromen-3-yl)-4H-furo[3,2-c]chromen-4-one, A4 = 8-chloro-3-(2-oxo-2H-chromen-3-yl)-4H-furo[3,2-c]chromen-4-one and x = 3, 2, 4, 1} were studied at room temperature. The fluorescence spectra of heterochelates show red shift, which may be due to the chelation by the ligands to the metal ion. It enhances ligand ability to accept electrons and decreases the electron transition energy. The kinetic parameters such as order of reaction (n), energy of activation (Ea), entropy (,S#), pre-exponential factor (A), enthalpy (,H#) and Gibbs free energy (,G#) have been reported. The antimicrobial activity of Clioquinol and Cu(II) heterochelates have been determined and described. All the heterochelates showed a more effective antimicrobial activity than the free ligand. Copyright © 2010 John Wiley & Sons, Ltd. [source] | |||||||||||||